Narrative:

We did the modification X to aircraft X and did the required resistance checks. The module and screws resistance checks to aircraft ground went well. The modules were grounded as required. There was a screw that grounding seemed high. With the rest of the resistance from the universal fault interrupter to aircraft ground being fine we went on. We just did the modification to aircraft Y and found the resistance to the small screw high. The small bracket was not grounded to the main module housing from the manufacturer. The universal fault interrupter units were grounded as were all the other screws to aircraft ground. There is no safety issue as the units are grounded through the main module mount and all other screws. Aircraft X was the first aircraft we did the modification to. We contacted engineering on the matter and they are in contact with tdg [manufacturer of ufi's]. Callback conversation with reporter revealed the following information: reporter stated his carrier is removing the B737-800 left and right relays at the P-91 and P-92 panels that allow power to the two center fuel tank boost pumps and replacing the relays with two universal fault interrupters (ufi)'south. One for each fuel boost pump. These interrupters are designed to monitor (read) the sudden electrical current draw that spikes when either of the center fuel tank pumps start to indicate low pressure. These ufi's will cut off electrical power to the center tank pump(south) sooner; instead of the pump(south) continuing to run in a low pressure situation until the pressure light on the overhead panel illuminates a low pressure signal for the pilot to switch the associated pump to off. Reporter stated his carrier hopes the ufi units will help reduce the high premature center fuel pump failures they have been experiencing on their B737-800's. Reporter also stated he does not know if the cockpit center fuel pump overhead low pressure light will come on whenever the ufi's trip power on any pump. These ufi's also record hard faults and can only be reset on the ground. Pilots will be able to recycle center fuel pump switches when pump electrical power trips in flight; as long as the ufi does not record the trip as a hard fault. Each ufi interrupter comes from the manufacturer in two pieces and is assembled on location in the lower east/east compartment; aft of the access door. Reporter stated that boeing has a similar system that is more complex; requires more wiring and is more time consuming to install.

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Original NASA ASRS Text

Title: AN AVIONIC'S MECHANIC REPORTS ON FINDING HIGH RESISTANCE AT A SMALL SCREW AND BRACKET ATTACHMENT AFTER INSTALLING UFI UNITS ON TWO B737-800 ACFT FOR THE CENTER TANK FUEL BOOST PUMPS.

Narrative: WE DID THE MODIFICATION X TO ACFT X AND DID THE REQUIRED RESISTANCE CHKS. THE MODULE AND SCREWS RESISTANCE CHKS TO ACFT GROUND WENT WELL. THE MODULES WERE GNDED AS REQUIRED. THERE WAS A SCREW THAT GROUNDING SEEMED HIGH. WITH THE REST OF THE RESISTANCE FROM THE UNIVERSAL FAULT INTERRUPTER TO ACFT GROUND BEING FINE WE WENT ON. WE JUST DID THE MODIFICATION TO ACFT Y AND FOUND THE RESISTANCE TO THE SMALL SCREW HIGH. THE SMALL BRACKET WAS NOT GROUNDED TO THE MAIN MODULE HOUSING FROM THE MANUFACTURER. THE UNIVERSAL FAULT INTERRUPTER UNITS WERE GROUNDED AS WERE ALL THE OTHER SCREWS TO ACFT GROUND. THERE IS NO SAFETY ISSUE AS THE UNITS ARE GROUNDED THROUGH THE MAIN MODULE MOUNT AND ALL OTHER SCREWS. ACFT X WAS THE FIRST ACFT WE DID THE MODIFICATION TO. WE CONTACTED ENGINEERING ON THE MATTER AND THEY ARE IN CONTACT WITH TDG [MANUFACTURER OF UFI'S]. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED HIS CARRIER IS REMOVING THE B737-800 LEFT AND RIGHT RELAYS AT THE P-91 AND P-92 PANELS THAT ALLOW POWER TO THE TWO CENTER FUEL TANK BOOST PUMPS AND REPLACING THE RELAYS WITH TWO UNIVERSAL FAULT INTERRUPTERS (UFI)'S. ONE FOR EACH FUEL BOOST PUMP. THESE INTERRUPTERS ARE DESIGNED TO MONITOR (READ) THE SUDDEN ELECTRICAL CURRENT DRAW THAT SPIKES WHEN EITHER OF THE CENTER FUEL TANK PUMPS START TO INDICATE LOW PRESSURE. THESE UFI'S WILL CUT OFF ELECTRICAL POWER TO THE CENTER TANK PUMP(S) SOONER; INSTEAD OF THE PUMP(S) CONTINUING TO RUN IN A LOW PRESSURE SITUATION UNTIL THE PRESSURE LIGHT ON THE OVERHEAD PANEL ILLUMINATES A LOW PRESSURE SIGNAL FOR THE PILOT TO SWITCH THE ASSOCIATED PUMP TO OFF. REPORTER STATED HIS CARRIER HOPES THE UFI UNITS WILL HELP REDUCE THE HIGH PREMATURE CENTER FUEL PUMP FAILURES THEY HAVE BEEN EXPERIENCING ON THEIR B737-800'S. REPORTER ALSO STATED HE DOES NOT KNOW IF THE COCKPIT CENTER FUEL PUMP OVERHEAD LOW PRESSURE LIGHT WILL COME ON WHENEVER THE UFI'S TRIP POWER ON ANY PUMP. THESE UFI'S ALSO RECORD HARD FAULTS AND CAN ONLY BE RESET ON THE GROUND. PILOTS WILL BE ABLE TO RECYCLE CENTER FUEL PUMP SWITCHES WHEN PUMP ELECTRICAL POWER TRIPS IN FLIGHT; AS LONG AS THE UFI DOES NOT RECORD THE TRIP AS A HARD FAULT. EACH UFI INTERRUPTER COMES FROM THE MANUFACTURER IN TWO PIECES AND IS ASSEMBLED ON LOCATION IN THE LOWER E/E COMPARTMENT; AFT OF THE ACCESS DOOR. REPORTER STATED THAT BOEING HAS A SIMILAR SYSTEM THAT IS MORE COMPLEX; REQUIRES MORE WIRING AND IS MORE TIME CONSUMING TO INSTALL.

Data retrieved from NASA's ASRS site as of May 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.